Embodiments of the present invention relate to a transition rate controlled bus driver circuit having reduced load capacitance sensitivity.
In wired digital communication systems with variable connection lengths and bus termination impedances there are challenging design limitations related to rise and fall time, power dissipation, and low and high output voltage levels. These limitations are further complicated by signal overshoot and inductive ringing which can cause interference between adjacent bus leads and communication errors. Frequently there must be a balance between bus circuit drive strength to control signal rise and fall times as well as to establish reliable logic levels after signal transitions. Some implementations may simply employ large n-channel and p-channel drive transistors with passive filter circuits, but these implementations are only effective for a limited range of bus loading. Other implementations may employ active current sources to achieve a controlled transition rate of a bus signal. However, these may be limited by power constraints.
Referring to FIG. 1, there is a simplified circuit diagram of a bus pull down circuit of the prior art. The circuit includes bus 100 and n-channel pull down transistor MPD 104. Feedback capacitor CPD 102 is coupled between the gate and drain of transistor MPD. In operation, the gate of MPD is driven high by current source 106 to pull bus lead 100 low while current source 108 is off. Alternatively, the gate of MPD is driven low by current source 108 when bus lead 100 is to remain high while current source 106 is off. Although this circuit provides a controlled pull down rate of bus lead 100, switching time is limited by current source 106 and threshold voltage and process variation of transistor MPD 104.
While preceding approaches have provided improvements in bus switching and power consumption, the present inventors recognize that still further improvements are possible. Accordingly, the preferred embodiments described below are directed toward improving upon the prior art.